The invention relates to timing control for high chiprate cellular CDMA systems, and more particularly to a method and apparatus for smoothly varying timing in response to either open loop or closed loop control mechanisms to correct for timing drift.
It is known in digital transceivers to derive both the transmit and receive frequencies from the same, accurate crystal reference oscillator, such as a voltage controlled oscillator (VCO). The standard known as the Global System for Mobile Communications (GSM) for digital cellular systems, for example, specifies that the network base stations shall lock the transmitted bitrate of 13 MHz/48 to the same frequency reference source as the radio frequency channels, which are multiples of 13 MHz/65. Accordingly, 13 MHz is a convenient choice for the reference oscillator frequency.
Mobile terminals use a technique known as automatic frequency correction (AFC) to lock their transmit and receive frequencies to the base station frequency reference. The mobile terminals receive and decode the base station transmissions and derive a frequency error representing the difference between the mobile terminal's frequency standard and the frequency of the received signal. The frequency error is then used to correct the mobile terminal's frequency standard by changing a control voltage applied to the VCO.
In many mobile terminals, the VCO from which all transmit and receive frequencies are derived is also used to generate all digital chiprates and bitrates. In time division multiple access (TDMA) systems, frame and slot timing is implicitly controlled since frame and slot periods are simply a specified number of bit periods. The bitrate/chiprate accuracy may need to be higher than radio channel frequency accuracy, in order to avoid the phenomenon of bitslip or mis-synchronization, particularly during periods of temporary signal outage as a result of fading.
Notwithstanding the use of the same frequency reference as both a frequency and timing standard, multipath fading due to motion of the mobile terminal causes Doppler-type frequency errors in the received signal. Due to multipath propagation, the multipath rays can be received from behind or in front of the mobile terminal relative to the direction of movement of the mobile terminal and therefore may be of any sign. As these multipath rays fluctuate, the frequency error perceived by the mobile terminal receiver fluctuates and becomes partially transferred to the crystal reference oscillator resulting in a small frequency error. This frequency error causes the timing to drift from an expected timing standard. For example, a mobile terminal operating according to the Universal Mobile Telecommunication System (UMTS) standard transmits at a chiprate of 3.84 megachips (complex QPSK chips) per second. Experience teaches that the crystal reference oscillator can be corrected by automatic frequency correction (AFC) to an accuracy of about 0.1 ppm. Thus, using the reference oscillator to derive the chiprate causes a timing drift of +/−0.384 chips/sec.
Another source of timing drift is relative motion. At 60 miles/hour vehicle speed, the loop path length is changing at +/−120 miles/hour, or 54 meters/sec. The loop delay is thus changing at +/−0.18uS/sec, which is +/−0.69 chips/sec. In total therefore, the timing drift can be up to +/−1 chip per second. The error induced by multipath fading and relative motion is typically corrected by two additional mechanisms: an open loop timing correction mechanism and a closed loop timing correction mechanism.
The open loop timing correction mechanism comprises determining, at the mobile terminal, the timing at which an assigned slot was received. Since the timing also fluctuates appreciably (several bit periods) due to the multipath propagation, the value is smoothed by a simple low pass filter having a smoothing time constant of several frames. The smoothed timing value is used to control the transmitter timing value in steps of ¼ of a bit period to follow the receiver timing by a specified number of bit periods, referred to herein as the transmitter timing offset.
The closed loop timing correction mechanism comprises the network base station receiving the mobile terminal signal and determining whether the slots are received late or early relative to an expected, ideal time-of-arrival. The base station then determines whether, on average, it is desirable to alter the transmitter timing offset between the mobile transmit and receiver timing. If so, a so-called time-advance command is transmitted from the network to the mobile terminal, thereby completing the closed loop correction mechanism.
In the GSM system, the mobile terminal may alter its transmitter timing according to the open loop mechanism in steps of ¼ of a bit period, or alter its transmit slot timing according to the closed loop mechanism in steps of more than one whole bit period. The timing offset is not critical in GSM. Because multipath propagation can change the propagation delays radically between one slot and the corresponding slot in the next frame, GSM receivers must in any case re-establish the channel propagation characteristics anew for each successive slot. There is thus no attempt to track the propagation channel changes between two successive frames.
In contrast to TDMA systems, code division multiple access (CDMA) systems generally employ continuous transmission, and attempt to track the channel changes due to multipath propagation, since there are no discontinuities at frame or slot boundaries to hinder channel tracking. Channel tracking allows more efficient receivers to be produced that operate at lower signal-to-noise plus interference ratios, which increases the capacity of CDMA systems. Advanced receivers using interference canceling techniques such as subtractive demodulation as described in U.S. Pat. No. 5,151,919 to Applicant, or coherent macrodiversity techniques as described in U.S. patent application Ser. No. 09/915,896, filed Jul. 26, 2001, entitled “Communication System Employing Transmit Macro-Diversity” to Applicant, are even more reliant on accurate channel tracking. If step changes are applied to mobile terminal transmitter timing, the accuracy of channel tracking is reduced, reducing the effectiveness of these advanced techniques.
The increase of CDMA bandwidths and chiprates has led to the need for further improvement in timing correction to prevent timing drift between a transmitter and a receiver.